The present invention relates to transient suppression devices, in general, and more particularly, to a transient suppression device (TSD) that is coupled in series with electrical wiring connecting a float switch disposed within a tank containing a combustible liquid and an interface circuit external to the tank, the TSD being located external to the tank for limiting to safe levels the current and energy of signals that may enter the tank through the electrical wiring while maintaining a drive current for operating the interface circuit.
An example of a float switch application in a combustible liquid is in connection with a transport aircraft on which one or more float switches is or are disposed within a fuel tank for monitoring the fuel level thereof. The float switch may comprise a magnetic reed switch that is sealed within a tube, for example. A float within the switch tube moves with the level of fuel and renders an electrical connection when the fuel rises above or drops below a predetermined fuel level. Each float switch is electrically connected to an interface circuit external to the tank by electrical wiring which passes through a wall of the tank. The electrical wiring interconnecting the float switch with its interface circuit is disposed within a sealed conduit which keeps the wiring from making contact with the fuel.
The float switch interface circuits vary in function with the aircraft. On some aircraft, the interface circuit acts as a safety device or back-up to prevent overfilling the tank during a fueling or re-fueling process. In this application, the interface circuit comprises a shut-off valve which typically uses currents on the order of two amps maximum, for example. On other aircraft, the interface circuit functions to initiate a fuel transfer between internal tanks of the aircraft. In this application, the interface circuit comprises a fuel transfer valve which typically uses currents on the order of sixty to one-hundred milliamps, for example. In some cases, the float switch may also drive a low fuel level indicator lamp. Such drive current levels are passed into the fuel tank through the interconnecting wiring and float switch.
Under normal operating conditions, a clear separation is maintained between the switch and wiring and the fuel to avoid the possibility of fuel vapor ignition. However, there are possible internal fault conditions in which fuel vapor may make contact with the wiring and/or switch contacts. For example, the conduit seal or the sealed tube or the connection between the conduit and tube may leak permitting fuel vapor to enter the sealed chambers. Other potential fault conditions include: chafing of the wires within the conduit resulting in potential exposure to fuel vapor; and hot short to the aircraft wiring. With the current and energy levels of the present float switch designs, any one of these fault conditions poses the risk of an explosive reaction. Another area of concern is with external threats, like lightning, high voltage wiring shorts, and high intensity radiated fields, for example, passing unsafe current and/or energy levels into the tank through the interconnecting switch wiring.
The Federal Aviation Administration or FAA has been considering these potentially threatening conditions and is on the verge of promulgating mandatory regulations to limit the current and energy levels permitted to pass into an aircraft fuel tank under any conditions, including normal operating conditions. These regulated levels of current and/or energy are intended to prevent an ignition of in-tank fuel vapors under all possible conditions. Under the anticipated FAA regulations, current levels of less than ten milliamps are considered safe, but current levels from ten to thirty milliamps will require an explanation of safeness. RMS current levels greater than thirty milliamps are considered unsafe under the anticipated regulations. In addition, the anticipated FAA regulations limit the energy permitted to enter the tank to less than two-hundred microjoules.
Accordingly, in order to comply with the anticipated FAA regulations, it may be necessary to modify the present float switch system on aircraft to reduce the normal in-tank operating current and energy of such systems while maintaining the current and energy needed to drive the interface electronics for all possible applications, and to protect against potentially threatening conditions as noted above. The present invention is intended to provide for these modifications.